1. Field of the Invention
The invention generally relates to the field of ultrasonic generators and, more particularly, to a system and method for heterodyning an ultrasonic signal to produce lower frequency signals.
2. Description of the Related Art
It is well known that ultrasonic generators and detectors can be used to locate leaks or defects, e.g., in pipes. Such a system is shown in U.S. Pat. No. 3,978,915 to Harris. In that arrangement, ultrasonic generators are positioned in a chamber through which the pipes pass. At the ends of these pipes, exterior to the chamber, ultrasonic detectors are located. At the point where a leak occurs in the pipe or the pipe wall is thin, the ultrasonic energy will enter the pipe from the chamber and travel to the end of the pipe where the detector is located. The detector will receive an ultrasonic signal at the end of the pipe indicating the existence of the leak or weak spot in the pipe.
By locating an ultrasonic generator in a closed chamber, a standing wave pattern with peaks and nodes is established. If a node occurs at the position of a leak or weak spot, no ultrasonic energy will escape and the defect will not be detected.
Ultrasonic sensors have also been used to detect ultrasonic energy generated by friction within mechanical devices as disclosed in U.S. Pat. No. Re. 33,977 to Goodman, et al., the details of which are hereby incorporated herein, in their entirety, by reference. The greater the amount of friction, the greater the intensity of the generated ultrasonic energy. Applying a lubricant to the device reduces friction and consequently the intensity of the generated ultrasound drops. Measuring ultrasonic energy thus provides a way to determine when lubrication has reached the friction generating surfaces. Additionally, faulty devices, such as bearings, generate a higher level of ultrasonic energy than do good bearings and thus, this condition can also be detected. However, conventional means require two people to perform this procedurexe2x80x94one person to apply the lubricant to the device, and one person to operate the ultrasonic detector.
In certain instances, e.g., when detecting the malfunction of bearings, an ultrasonic detector is mechanically coupled to the casing of the bearings so that the vibrations caused by the malfunction can be mechanically transmitted to it. With such an arrangement, the frequency is not set by an ultrasonic generator, but is created by the mechanical vibration itself. Here, an ultrasonic detector circuit must be capable of sweeping over a band of frequencies to locate the one frequency that is characteristic of the malfunction. This is usually accomplished by a heterodyning circuit which can be tuned to various frequencies, much in the manner of a radio receiver.
Since ultrasonic energy used for these purposes is generally in the range of 40 kHz, it is too high in frequency to be heard by a human being. Thus, means are typically provided for heterodyning, or frequency shifting, the detected signal into the audio range, and various schemes are available for doing this.
Ultrasonic transducers generally produce a low voltage output in response to received ultrasonic energy. Thus, it is necessary to amplify the detected signal using a high-gain preamplifier before it can be accurately processed. However, if low cost heterodyning and display circuitry are to be used, means must be made available to attenuate the amplified signal to prevent saturating these circuits when high input signals are present. This attenuation also adjusts the sensitivity of the device. For a hand-held unit, the degree of attenuation should be selectable by the user. For example, U.S. Pat. No. 4,785,659 to Rose et al. discloses an ultrasonic leak detector with a variable resistor attenuator used to adjust the output level of an LED bar graph display. However, this attenuation method does not provide a way to establish fixed reference points to allow for repeatable measurements.
U.S. Pat. No. 5,089,997 to Pecukonis discloses an ultrasonic energy detector with an attenuation network positioned after an initial pre-amplifier and before the signal processing circuitry, which creates an audible output and an LED bar graph display. The resistors in the Pecukonis attenuation network are designed to provide an exponential relationship between the different levels of attenuation. However, Pecukonis does not heterodyne the detected signals to produce an audible output, but rather teaches the benefits of a more complex set of circuits which compress a broad range of ultrasonic frequencies into a narrower audible range. For many applications, the cost and complexity of this type of circuitry are not necessary.
When using ultrasonic energy to detect leaks, it is useful to have a portable ultrasonic sensor which indicates the presence and intensity of ultrasonic energy both visually and audibly. U.S. Pat. No. Re. 33,977 to Goodman et al. discloses an ultrasonic sensor that displays the intensity of the detected signal on an output meter operable in either linear or logarithmic mode, and also provides for audio output through headphones. U.S. Pat. No. 4,987,769 to Peacock et al. discloses an ultrasonic detector that displays the amplitude of the detected ultrasonic signal on a ten-stage logarithmic LED display. However, the detector disclosed in Peacock does not process the detected signal to produce an audible response, nor does it provide for signal attenuation after the initial pre-amplification stage.
Means have been proposed for increasing the output of the ultrasonic transducer. For example, in U.S. Pat. No. 3,374,663 to Morris it is suggested that an increase in the voltage output can be achieved by serially arranging two transducers. It has been found, however, that with such an arrangement a typical transistor pre-amplifier loads the transducers to such an extent that the gains achieved by stacking them serially are lost. The Morris patent proposes the use of a triple Darlington configuration in order to produce a sufficiently high input impedance to prevent this degradation in the signal produced by the stack of transducers. Unfortunately, the transducers in this arrangement are not placed so that they both readily receive ultrasonic energy. Thus, the Morris arrangement is not entirely satisfactory.
The present invention is directed to providing improved methods and apparatus for detecting leaks and mechanical faults by ultrasonic means. In accordance with the invention, an input transducer signal is applied to a unity gain buffer amplifier that is used to maintain the impedance level seen by the transducer. The processed signal from the unity gain buffer amplifier is supplied to a voltage control amplifier that also receives a voltage control signal that is generated by a digital-to-analog converter located on an external I/O board. The voltage control signal is used to switch the voltage controlled amplifier such that the dynamic range of the signal is expanded prior to a clip of the signal. The voltage control signal is based on a level that is programmed into the voltage control amplifier by the digital-to-analog converter located on the external I/O board. The voltage controller is thus controlled by the I/O board in response to commands sent to the external I/O board from a micro-controller.
The output from the voltage controlled amplifier is connected to a fixed gain differential amplifier. The output signal from the fixed gain amplifier is supplied to a variable gain amplifier that is switchable between two fixed levels, such as 0 dB and 20 dB. The gain level of the variable gain amplifier is toggled between the two fixed gain levels based on a level that is determined by the amount of gain that is programmed into the voltage control amplifier.
The output of the variable gain amplifier is supplied to a pair of heterodyning circuits, i.e., a dual heterodyning circuit. At each respective heterodyning circuit, the output signal from the variable gain amplifier is multiplied with a local oscillator signal that is internal to each circuit. Here, each local oscillator is nominally set to 38 kHz such that for a 40 kHz input transducer signal, a difference frequency of about 2 kHz (i.e., the audio component) is provided at the output of each heterodyning circuit.
The output signal from the first heterodyning circuit is amplified and divided into two signal branches. The first signal branch is transformer coupled to a headphone output. The second signal branch is connected to an amplifier that is also transformer coupled to a line output and also applied to an external audio amplifier. The output from the second of the heterodyning circuits is amplified and supplied to a metering circuit.
In addition, a further analog signal path is created at the second heterodyning circuit. The signal in this path is converted to a linear dB format analog signal and supplied to a micro-controller. This analog signal is converted in the micro-controller into a digital signal by an analog-to-digital converter, and is further converted in the micro-controller into a WAV file format, as well as other digital signal formats, for subsequent spectral analysis.
The present inventors have determined that a heterodyned signal that drives a meter requires a relatively large dynamic range, but a limited frequency response, while a heterodyned signal that is required for headphones or spectral analysis may have a low dynamic range, but requires high resolution. Further, it has been found that the resolution or frequency response of the input transducer signal is degraded if a single heterodyning circuit is used to drive a number of circuits or meters with competing requirements. In order to overcome these competing requirements, the present invention uses a dual heterodyning circuit in which the two individual heterodyne circuits are separately optimized so that the second results in a signal with a large dynamic range and the first results in a signal with a great resolution, and neither unduly loads the transducer array or obscures subtle frequency components. This permits the capture of particularly low level frequency components for extraction during spectral analysis.
In accordance with the invention, the first heterodyning circuit has a feed back loop filter and a transformer to provide an enhanced spectral (i.e., frequency) response. This circuit is used to drive the headphone, a wave file generator and a line output. This signal, which has a modest dynamic range but a high frequency response and a low signal to noise ratio, allows the spectrum of the signal to be analyzed in real time by an external spectrum analyzer, recorded for later analysis or listened to in real time through the headphones.
The second heterodyning circuit has a smaller frequency response but a larger dynamic range so that it can drive the meter. In accordance with the invention, the second heterodyne circuit is not required to have an optimized spectral response. If the meter were driven with the first heterodyne circuit, the impedance and dynamic range requirements of the meter would adversely affect the response. Thus, two heterodyne circuits are used, with the circuit that drives the meter being simpler, and less costly to manufacture and having a larger dynamic range.
In either mechanical analysis or electrical equipment analysis, a large number of frequencies in the low frequency range become lost. This is especially true in the case of electrical applications. After extended use of the detection equipment, operators often tend to begin to use their ears as a guide to the condition of an area of concern. However, it is extremely difficult for a person to discern with their ears the differences between inputs that are electrical in nature and inputs that are vibrational. Further, in other technologies, such as vibration analysis, infrared technologies, or where rotational equipment is used, the use of the human ear is a highly unreliable way in which to predict faults. For example, a transformer resonating at 60 Hz may cause a component in an equipment cabinet to resonate at the same 60 Hz. When an operator listens to the cabinet containing the component that is vibrating at the 60 Hz, it is impossible to determine whether the resonance is electrical or mechanical.
By using the dual heterodyning circuit of the present invention to provide the enhanced spectrum, it becomes clear whether a detected resonance is mechanical or electrical. In addition, fault frequencies are also more easily discernable. In other words, the enhanced signal output provides a lower signal-to-noise ratio, so as to increase the ease with which frequency components are analyzed.